For example, Marks et al. describe components produced using a precursor-containing composition comprising the salt InCl3 and the base monoethanolamine (MEA) in solution in methoxyethanol. Following spin coating of the composition, the corresponding indium oxide layer is produced by thermal treatment at 400° C. (H. S. Kim, P. D. Byrne, A. Facchetti, T. J. Marks; J. Am. Chem. Soc. 2008, 130, 12580-12581 and supplemental information).
As compared with compositions containing indium salt, compositions containing indium alkoxide and/or indium halogen alkoxide offer the advantage that they can be converted into indium oxide-containing coatings at lower temperatures. Moreover, it was hitherto assumed that halogen-containing precursors potentially had the disadvantage of leading to halogen-containing layers of reduced quality. For this reason, in the past, experiments on layer formation were performed using indium alkoxides.
Indium alkoxides and indium halogen alkoxides, and their synthesis, were described from as early as the 1970s.
For example Carmalt et al., in a review article, summarized the data known up until that point in time concerning synthesis, structure and reactivities of, among other compounds, indium(III) alkoxides and alkylalkoxides (Carmalt et al., Coord. Chem. Rev. 250 (2006), 682-709).
One of the oldest known syntheses of indium alkoxides is described by Chatterjee et al. They describe the preparation of indium trisalkoxide In(OR)3 from indium(III) chloride (InCl3) with sodium alkoxide NaOR, with R standing for methyl, ethyl, isopropyl, n-butyl, sec-butyl, tert-butyl and pentyl radicals (S. Chatterjee, S. R. Bindal, R. C. Mehrotra; J. Indian Chem. Soc. 1976, 53, 867).
Bradley et al. report on a similar reaction to that of Chatterjee et al., and with approximately identical reactants (InCl3, isopropyl-sodium) and reaction conditions, obtain an indium oxo alkoxide cluster with oxygen as the central atom (D. C. Bradley, H. Chudzynska, D. M. Frigo, M. E. Hammond, M. B. Hursthouse, M. A. Mazid; Polyhedron 1990, 9, 719).
One particularly good variant of this process, leading to a particularly low level of chlorine contamination in the product, is described in US 2009-0112012 A1. The efforts to achieve as low as possible a level of chlorine impurities in the product are attributed there to the previous assumption that chlorine impurities contribute to a reduction in the performance or lifetime of electronic components (compare, for example, U.S. Pat. No. 6,426,425 B2).
Likewise based on an indium halide, but on different bases, is the process U.S. Pat. No. 5,237,081 A described for preparing pure indium alkoxides, by reacting an indium(III) halide in a basic medium with an alcohol. The base is said to be a strong base with low nucleophilicity. Bases exemplified, as well as complex cyclic heterocycles cited by way of example, include tertiary amines.
Alternative synthesis routes to homoleptic indium alkoxide complexes are described by Seigi Suh et al. in J. Am. Chem. Soc. 2000, 122, 9396-9404. The processes described therein, however, are very costly and inconvenient, and/or are based on reactants which are not available commercially (and must therefore, as a disadvantage, be synthesized in a step beforehand).
A general process for preparing halogen-alkoxy-metal compounds is described in U.S. Pat. No. 4,681,959 A: Described generally therein is a two-stage process for preparing metal alkoxides (especially tetraalkoxy compounds such as tetramethyl titanate), by reacting a halide of at least divalent metal with an alcohol—optionally in the presence of an aromatic solvent—initially to form an intermediate (a halogen-alkoxy compound of the metal). Hydrogen halide formed in the course of this reaction is preferably driven out using an inert gas such as nitrogen, for example.
Indium halogen alkoxides and their synthesis are also described in JP 02-113033 A and JP 02-145459 A. Accordingly, JP 02-113033 A discloses how chlorine-containing alkoxides of indium can be prepared, following dissolution of indium chloride in an alcohol corresponding to the alkoxide radical to be installed, by subsequent addition of a particular fraction of an alkali metal or of an alkali metal alkoxide. A process of this kind is also described by JP 02-145459 A.
The preparation of indium oxide-containing layers from indium alkoxides and indium halogen alkoxides may take place in principle i) by sol-gel procedures, in which the precursors used react in the presence of water, by hydrolysis and subsequent condensation, initially to form gels, and are then converted into metal oxides, or ii) from nonaqueous solution. The conversion to indium oxide-containing layers may take place thermally and/or by electromagnetic radiation.
Processes for preparing indium oxide-containing layers by thermal conversion are part of the prior art. WO 2008/083310 A1, for example, describes processes for producing inorganic layers or organic/inorganic hybrid layers on a substrate by applying to said substrate a metal oxide (e.g. one of the generic formula R1M (OR2)y-x), or a prepolymer thereof, and then curing the resultant metal oxide layer in the presence of water, with reaction with water, while supplying heat. Among the metal alkoxides which can be used is an indium alkoxide. Following the conversion, the layer obtained may be treated subsequently with heat or UV.
JP 01-115010 A as well is concerned with a thermal conversion in a sol-gel procedure. This document describes compositions for transparent, thin, conducting layers that have a long pot life, do not, as a composition, undergo hydrolysis, and comprise chlorine-containing indium alkoxides of the formula In(OR)xCl3-x. These compositions can be converted following application to a substrate, gelling of the alkoxide on the substrate by the water fraction in air, and subsequent drying at up to 200° C., with conversion taking place at temperatures of 400-600° C.
JP 02-113033 A describes processes for applying an antistatic coating to a non-metallic material, by coating the non-metallic material with a composition comprising a chlorine-containing indium alkoxide, gelling the composition in air, and then calcining it.
JP 2007-042689 A describes metal alkoxide solutions, which can comprise indium alkoxides, and also processes for producing semiconductor components using these metal alkoxide solutions. The metal alkoxide solutions can be converted to form the oxide layer by means of a thermal treatment.
JP 02-145459 A describes coating compositions which contain indium halogen alkoxide, which do not undergo hydrolysis in the course of storage, and which can be converted, by calcining, into an indium oxide-containing layer.
JP 59-198607 A describes processes for producing transparent conducting layers which can have a protective film of various resins. The transparent conducting layer may be an indium oxide-containing layer, and may be produced via a liquid phase process in which a corresponding composition is applied to a substrate, dried, and converted thermally. According to the examples, a composition comprising InCl(OC3H7)2 can be used.
JP 59-198606 A describes compositions for the formation of transparent, electrically conducting layers, comprising InClx(OR)3-x and an organic solvent and having a water fraction of 0.1%-10%, based on the organic solvent. Such a composition, accordingly, is a sol of an indium halogen alkoxide. To form the transparent conducting layer, the composition is applied to a substrate and dried at typically 150° C., and then baked at temperatures of preferably 300° C.
Conversion carried out by means of heat alone, however, has the disadvantage that it cannot be used to produce fine structures and that, furthermore, it does not allow precise control over the resulting layer properties.
For this reason, processes for converting to indium oxide-containing layers were developed which are based on the use of electromagnetic radiation (especially UV radiation).
For instance, JP 09-157855 A describes a sol-gel process for producing a metal oxide layer, by applying to the surface of a substrate a metal oxide sol prepared by hydrolysis and composed of a metal alkoxide or a metal salt (e.g. an indium alkoxide or indium salt), said application taking place optionally at a temperature at which the gel still does not crystallize, and being followed by drying and by UV irradiation at less than 360 nm.
Conversions which take place exclusively by radiation, however, have the disadvantage that they require very high energy densities over a relatively long time and are therefore very costly and inconvenient in terms of apparatus. For this reason, processes were developed which are based not only on thermal conversion but also on conversion using electromagnetic radiation.
JP 2000-016812 A describes a process for producing a metal oxide layer via a sol-gel process. In this process, the substrate is coated with a coating composition of a metal oxide sol comprising a metal salt or metal alkoxide, more particularly an In2O3—SnO2 composition, and the coating is irradiated with UV radiation at a wavelength of less than 360 nm, and subjected to heat treatment.
JP 11-106935 A describes a process for producing an oxide-based transparent conducting film, in which inter alia an anhydrous composition comprising an alkoxide of a metal (e.g. indium) is applied to a substrate and heated. The film may also be subsequently converted using UV or VIS radiation into a metal oxide-based thin layer.
DE 10 2009 054 997 describes liquid phase processes for producing indium oxide-containing layers from non-aqueous solution, by applying an anhydrous composition comprising at least one solvent or dispersion medium and at least one precursor of the formula InX(OR)2 to a substrate in an anhydrous atmosphere and subjecting it to irradiation with electromagnetic radiation <360 nm and to thermal conversion.
The known processes for producing indium oxide-containing layers via combined thermal conversion and conversion with electromagnetic radiation, however, have the disadvantage that the use of the indium alkoxides described exhaustively in the literature exhibits much poorer semiconducting properties. Furthermore, even the combination of thermal conversion with conversion by means of electromagnetic or UV radiation alone does not result in sufficiently satisfactory results, particularly in relation to the resultant field-effect mobilities μFET. The problem posed, therefore, is that of overcoming the outlined disadvantages of the prior art and of providing an improved process for producing indium oxide-containing layers.